This invention relates to an exhaust gas component concentration sensing device provided in a fuel supply control system for an internal combustion engine in which the air-fuel ratio of an air-fuel mixture supplied to the engine is controlled in response to the difference between the output voltage of the sensing device and a predetermined reference voltage, and a method of detecting failure of the sensing device, which detects disconnection or short-circuit in the sensing device from change in the output voltage thereof.
The exhaust gas component concentration sensing device has an improved construction which enables accurate and positive detection of failure thereof by the use of the failure detecting method of the invention.
In general, in order to control the air-fuel ratio of a mixture supplied to an internal combustion engine within a certain range having a desired value as its central value, concentration of a particular component contained in the exhaust gases, such as oxygen, is detected by a sensor, and responsive to the detected oxygen concentration, an air-fuel ratio correction coefficient is set to a proper value to thereby correct the air-fuel ratio of the mixture. The sensor for detecting the oxygen concentration in the exhaust gases, i.e. a so-called O.sub.2 sensor is formed, for instance, of a solid electrolytic element of zirconia (ZrO.sub.2), which has the characteristic that the electromotive force thereof sharply changes as the air-fuel ratio of the mixture changes across the stoichiometric ratio. The output voltage from the O.sub.2 sensor is at a high level when the air-fuel ratio is richer than the stoichiometric ratio and at a low level when the air-fuel ratio is leaner than same. Failure of the O.sub.2 sensor, such as disconnection and short-circuit, and deterioration thereof seriously affect the air-fuel ratio control. Therefore, it is necessary to always monitor an exhaust gas component concentration sensing device including the O.sub.2 sensor so as to enable the air-fuel ratio control system to normally function based on a normal output signal from the sensing device.
To this end, a method of detecting failure of an exhaust gas component concentration sensor (O.sub.2 sensor) has been proposed e.g. by Japanese Patent Publication (Kokoku) No. 56-29100, in which a time interval at which inversion of the output voltage of the O.sub.2 sensor takes place, i.e. the time interval between inversion of the output voltage from the high level to the low level and vice versa, is detected to thereby determine whether the O.sub.2 sensor is faulty. Another method has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 53-95431, in which the actual output voltage from the O.sub.2 sensor is compared predetermined voltage higher than the upper limit of output voltage range (approximately 0.1 V to 1 V) that can be assumed during normal functioning of the O.sub.2 sensor to thereby determine whether the O.sub.2 sensor is faulty.
According to the former method, the time interval of inversion of an output voltage from a comparator to which the output from the O.sub.2 sensor is applied is measured from an amount of charge accumulated in a capacitor. If the time interval is longer than a predetermined time interval, it is determined that the O.sub.2 sensor is faulty. However, in this method, if a basic air-fuel ratio (e.g. corresponding to the basic value Ti of the fuel injection period T.sub.OUT) is deviated from a proper value, the sensor output can stay on the lean side or rich side with respect to the stoichiometric air-fuel ratio over the predetermined time interval leading to an erroneous judgement that the O.sub.2 sensor is faulty.
Specifically, if predetermined values of the basic value Ti of the fuel injection period stored in a map are deviated from proper values actually required by the engine, or if the opening area of a fuel injector or set fuel pressure from a fuel pressure regulator of the engine is deviated from a proper value due to manufacturing variations or aging even if the map values are proper, the air-fuel ratio may stay on the rich side or lena side even after a time period sufficient for inversion of the output voltage of the O.sub.2 sensor has elapsed.
Thus, the proposed method of detecting the time interval of inversion of the output of the O.sub.2 sensor for detecting failure of same may erroneously detect failure of the O.sub.2 sensor in the above cases, even though the sensor is not faulty. In other words the proposed method is susceptible to deviation of the basic air-fuel ratio and variations etc. in component parts of the engine, which affects the accuracy and reliability of the failure detection.
On the other hand, according to the latter method in which it is determined that the O.sub.2 sensor is faulty when the output voltage from the sensor is above a predetermined high voltage, e.g. 6 V, although it is able to detect disconnection in the O.sub.2 sensor or its wiring, it is unable to detect short-circuit in the sensor or its wiring since the sensor output voltage drops to zero when the O.sub.2 sensor or its wiring is short-circuited.